In today's computing environments, many computers can have access to many areas of storage on one or several storage subsystems. The areas of storage on disks, CD-ROMs and the like are usually arranged into file systems consisting of organized directories and files for ease of access by the user of a given computer. In many applications, it is useful to be able to share such file systems between multiple computers.
Software is available to enable such file sharing for different types of computing systems. For example, the Network File System (“NFS”), developed by Sun Microsystems Inc., allows sharing of file systems in such computing environments. NFS allows a server, known as the NFS server, to “export” a file system such that it is available for use by other computers, known as NFS clients. The NFS clients can then “mount” the file system, allowing them to access the file system through the NFS server that exported the file system.
Many computing environments implement critical software, such as database applications, that must be highly available. That is, little down time can be tolerated for these applications. Such critical software often requires a file sharing system. NFS, though simple to implement, presents a single point of failure for a given file system because all the NFS clients access the file system through the NFS server. NFS is therefore not an ideal file sharing system in its present form for use in a highly available environment.
Highly available file systems, known as “clustered file systems” (“CFS”) have been developed in order to provide a more robust file system sharing solution. A typical CFS allows storage devices such as disks, CD-ROMs and the like to be attached to all nodes in the cluster and to be accessed directly by every node. The storage devices appear to be controlled and managed by a single entity, which continues to operate as nodes in the cluster fail and recover. Since all nodes have direct access to all storage, there is no single point of failure. Though a CFS provides a highly available shared file system, a CFS is very complicated, expensive, and difficult to implement.
There is a need for a shared file system that is highly available but relatively simple to implement and that avoids the other inadequacies and disadvantages associated with implementing a full scale CFS.